It's not the river we once knew

BROWN DRAIN: Water runs down the spillway into the pond below the Maitai Dam.

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Nelson built a dam and that's where our water comes from, right? Wrong, actually.

For sure, the 32-hectare Maitai reservoir holds plenty of water - 4 million cubic metres when full. But it's mainly used to keep the Maitai running the 13 kilometres from the dam to the sea at an acceptable flow level. Most of the time the city's water comes from the river's South Branch, through an intake near the dam.

When the river floods and gets discoloured, the reservoir is used temporarily. It also cuts in at times of peak summer demand when the South Branch can't provide enough.

Some water from the Roding River is also used to supply Nelson.

But for the most part, the dam is holding water to maintain the Maitai at a minimum flow of 300 litres a second from April to October and 175 l/sec from November to March, as required by the Nelson City Council's resource consents.

Take a drive to the foot of the dam - a 20-minute trip from the city - and walk for 800 metres up the South Branch and you can see where its gin-clear headwaters meet the tannin-stained water piped from the dam.

That's because the North Branch which feeds the dam runs through the type of native forest that produces the discolouration familiar in many yellowed West Coast rivers. The South Branch doesn't. There's a marked difference in not only the colour of the water, but the river bed. Above the point where the dam water arrives, the rocks and gravel look clean. Below it, they're coated with a dark brown slime.

It's not the staining of the water that is a concern, but that when it is released from the reservoir's lower layer it is lacking in oxygen and can produce changes in water chemistry thought to be favourable for cyanobacteria, the blue-green algae that has killed dogs and poses a threat to people when it becomes toxic.

The reservoir water has three distinct layers; in mid to late summer the bottom layer contains no oxygen and in a report for the council, scientists from Nelson's Cawthron Institute have suggested oxygenating it with a system that could cost under $70,000 and with annual running costs of around $20,000.

Whether or not that idea takes hold, the council's resource consents expire in 2017, prompting a flurry of reports into the health of both the reservoir water and the upper, middle and lower reaches of the Maitai.

These map the history of the water scheme, study the various life-forms that inhabit the river, suggest reasons for its proven degradation and put up a series of possible changes to improve what is generally accepted as a river that isn't what it used to be, but compares well to other urban waterways around New Zealand.

Nelson's first water scheme was on the Brook, a Maitai tributary. It was commissioned in 1867 and by the 1930s it was insufficient for the growing city's needs.

A weir on the Roding River was put in first, partly because it benefited not only the city but also Tahunanui, which had its own town board, Stoke, which was then part of the Waimea County, and Richmond, then a borough.

This was only a temporary solution. A Maitai dam was proposed in the 1950s and a pipeline was completed in 1963 but the dam was postponed in favour of a "temporary" intake grating on the South Branch.

As recorded in the Prow, the top of the south's historical and cultural council-led website, problems with both volume and quality brought fresh calls for a dam in the 1970s.

Many reports and much fierce debate later, the 36m-high earth dam was begun. It was completed in 1987 and cost $9.7 million - a huge investment for the city at the time. The Health Department gave $1m, the council borrowed the rest, and by 2005 the loans were repaid.

But water quality remained an issue and with "boil water" warnings necessary after heavy rain in the Maitai catchment and growing government attention to the health of water supplies around New Zealand, the council commissioned a hi-tech membrane micro-filtration plant. Finished in August 2004, it cost $26m - $10m more than first thought.

Built in the hills behind the Brook Valley, it treats the water from the Maitai and Roding rivers, which is then supplied to all of the city's urban and rural areas, from Saxton Field to the Glen. It produces level A water, classified as "completely satisfactory, extremely low level of risk". The distribution system, much of which is old and being upgraded, is rated B, "satisfactory, very low level of risk".

But what of the river?

The Maitai was one of New Zealand's earliest fisheries open to licence-holders and because it flows through the centre of Nelson and held an abundant supply of small trout, it was long a popular destination for trout anglers. Some, like fishing guide and Nelson Mail outdoors columnist Zane Mirfin, blame the dam for the decline in the fish population.

"The Maitai introduced four or five generations of Nelson anglers to the fine art of trout fishing and was highly rated as a small trout fishery for well over a century before meeting its ‘waterloo' when the Nelson City Council built the Maitai Dam," he wrote in Fish & Game magazine.

But in a recent Mail column he reported that he'd fished the river again after a long break and was surprised by how many fish he caught and released.

A stealthy walk up any stretch of the river will show that it does hold trout of varying size. Eels and native fish can also be spotted.

The Maitai's decline is not terminal, and it is clear that the dam and the water from the reservoir behind it are just one factor in what has degraded it.

"Plantation forestry and urban stormwater runoff appear to be the dominant pressures facing the Maitai catchment," a Cawthron report concludes.

Yet its lower reaches are polluted enough by sewage for a "no swimming" warning to remain for years, and the occasional dangerous toxicity of the ever-present cyanobacteria is a continuing serious concern, and possibly tied to the dam water.

These issues and their management implications are central to the council's difficulties as the expiration of the water scheme's resource consents draws closer.